I think the E510 dates from the early 2000s, meaning there's no way the system has a UEFI boot option, further meaning the drive is in MBR format -- which cannot be used as a boot device in a UEFI-only system like the 8960.  You'd need to wipe and re-initialize the hard drive in GPT layout to use it as a boot device in your new desktop system. 
I bought an XPS 8960 to replace a very old Dell Dimension E510 running Win10 on a 2TB HDD. I would like to dual boot the XPS to its stock NVMe SSD (drive C:) Win11 or to the old HDD from the E510. So I installed the old HDD into the XPS (this showed up as an F: drive in Win11, NTFS, Healthy, IFS partition, Boot volume. It works fine as a data disk). I used bcdboot F:\Windows to create a boot menu for this drive, but when I try to boot from this drive I get the following error: "a required file is missing or contains errors" 0xc0000359 \Windows\system32\winload.efiI tried booting from a Win10 USB stick (secure boot off) thinking I might be able to recover Win10 on the old HDD, but it could not see any of the two HDDs on the XPS. Not sure what to try next. Any ideas?
XPS 8960, dual boot to SSD or HDD