user60541: After having finished the "Initial Block Download" I needed to reindex:
[BTC_Core_Expmnt]# bitcoind -reindex -datadir=/mnt/BTC_Core_Expmnt

The debug.log showed everything starting up ok; and the process beginning:
[2020-01-20T07:43:41Z Loaded 119369 blocks from external file in 6005ms\
2020-01-20T07:43:41Z Reindexing block file blk00001.dat...]

Then a while later this:
[2020-01-20T09:05:59Z Reindexing finished]

But bitcoind continued grinding on... Well then what is all this? ::
[2020-01-20T09:05:59Z Pre-allocating up to position 0x100000 in\
[rev00000.dat 2020-01-20T09:05:59Z UpdateTip: new best=\ 
[00000000839a8e6886ab5951d76f411475428afc90947ee320161bbf18eb6048\
[height=1 version=0x00000001 log2_work=33.000022 tx=2\
[date='2009-01-09T02:54:25Z' progress=0.000000 cache=0.0MiB(1txo)]

In any case it got to this point ::
[2020-01-20T09:59:00Z UpdateTip: new best=\
[00000000000000001088ed0c0989828504a41d4b5b974c8c081b708cfdd3b7b0\
[height=359645 version=0x00000003 log2_work=82.893764 tx=71128141\
[date='2015-06-06T07:25:10Z' progress=0.143166\
[cache=669.7MiB(4972713txo)]

Then it fell over ::
[2020-01-20T09:59:00Z *** Disk space is too low!
[2020-01-20T09:59:00Z Error: Disk space is too low!\
[Error: Disk space is too low!
[2020-01-20T09:59:00Z Failed to connect best block\
[ (Disk space is too low! (code 0)) ]

Which is odd, because there should still be 20GB free since the size of
BTC_Core_Expmnt is supposed to be 300GB.
[BTC_Core_Expmnt]# du -h
102M    ./blocks/index
275G    ./blocks
56K     ./bitcoin-0.19.0.1/share/man/man1
60K     ./bitcoin-0.19.0.1/share/man
64K     ./bitcoin-0.19.0.1/share
73M     ./bitcoin-0.19.0.1/bin
8.0K    ./bitcoin-0.19.0.1/include
440K    ./bitcoin-0.19.0.1/lib
74M     ./bitcoin-0.19.0.1
16K     ./lost+found
3.4G    ./indexes/txindex
3.4G    ./indexes
914M    ./chainstate
280G    .
[BTC_Core_Expmnt]#

Is 300GB not enough in 2020 to run a full node?  I thought it'd do.  But, asking for any helpful tips because I cannot see anything else amiss from debug.log.

user48884: It refers to actual disk space. RAM is memory, not storage. The term "disk space" is very specific.

Then a while later this:
[2020-01-20T09:05:59Z Reindexing finished]

But bitcoind continued grinding on... Well then what is all this? ::

Your node was not fully synced, so after it finished reindexing the blocks on disk, it continued to download blocks from its peers and finish syncing.

60541: <p>After having finished the "Initial Block Download" I needed to reindex:</p>

<pre><code>[BTC_Core_Expmnt]# bitcoind -reindex -datadir=/mnt/BTC_Core_Expmnt
</code></pre>

<p>The debug.log showed everything starting up ok; and the process beginning:</p>

<pre><code>[2020-01-20T07:43:41Z Loaded 119369 blocks from external file in 6005ms\
2020-01-20T07:43:41Z Reindexing block file blk00001.dat...]
</code></pre>

<p>Then a while later this:</p>

<pre><code>[2020-01-20T09:05:59Z Reindexing finished]
</code></pre>

<p>But bitcoind continued grinding on... Well then what is all this? ::</p>

<pre><code>[2020-01-20T09:05:59Z Pre-allocating up to position 0x100000 in\
[rev00000.dat 2020-01-20T09:05:59Z UpdateTip: new best=\ 
[00000000839a8e6886ab5951d76f411475428afc90947ee320161bbf18eb6048\
[height=1 version=0x00000001 log2_work=33.000022 tx=2\
[date='2009-01-09T02:54:25Z' progress=0.000000 cache=0.0MiB(1txo)]
</code></pre>

<p>In any case it got to this point ::</p>

<pre><code>[2020-01-20T09:59:00Z UpdateTip: new best=\
[00000000000000001088ed0c0989828504a41d4b5b974c8c081b708cfdd3b7b0\
[height=359645 version=0x00000003 log2_work=82.893764 tx=71128141\
[date='2015-06-06T07:25:10Z' progress=0.143166\
[cache=669.7MiB(4972713txo)]
</code></pre>

<p>Then it fell over ::</p>

<pre><code>[2020-01-20T09:59:00Z *** Disk space is too low!
[2020-01-20T09:59:00Z Error: Disk space is too low!\
[Error: Disk space is too low!
[2020-01-20T09:59:00Z Failed to connect best block\
[ (Disk space is too low! (code 0)) ]
</code></pre>

<p>Which is odd, because there should still be 20GB free since the size of
BTC_Core_Expmnt is supposed to be 300GB.</p>

<pre><code>[BTC_Core_Expmnt]# du -h
102M    ./blocks/index
275G    ./blocks
56K     ./bitcoin-0.19.0.1/share/man/man1
60K     ./bitcoin-0.19.0.1/share/man
64K     ./bitcoin-0.19.0.1/share
73M     ./bitcoin-0.19.0.1/bin
8.0K    ./bitcoin-0.19.0.1/include
440K    ./bitcoin-0.19.0.1/lib
74M     ./bitcoin-0.19.0.1
16K     ./lost+found
3.4G    ./indexes/txindex
3.4G    ./indexes
914M    ./chainstate
280G    .
[BTC_Core_Expmnt]#
</code></pre>

<p>Is 300GB not enough in 2020 to run a full node?  I thought it'd do.  But, asking for any helpful tips because I cannot see anything else amiss from debug.log.</p>
 48884: <p>It refers to actual disk space. RAM is memory, not storage. The term "disk space" is very specific.</p>

<blockquote>
  <p>Then a while later this:</p>

<pre><code>[2020-01-20T09:05:59Z Reindexing finished]
</code></pre>
  
  <p>But bitcoind continued grinding on... Well then what is all this? ::</p>
</blockquote>

<p>Your node was not fully synced, so after it finished reindexing the blocks on disk, it continued to download blocks from its peers and finish syncing.</p>
