Hello Steen, There are no issues with LTO5 media working in the TL4000.  What I would suggest is that since you can use the LTO4 media & it is a different manufacture then the LTO5 media & your LTO4 media works in the LTO5 drives without any issue is to check with the manufacture of the LTO5 media to confirm that there are no issues with your media.  If both your LTO4 & 5 media gave the same errors on all media then I would say you have a drive issue but due to it being with just one manufacture media doing it then it has to be the LTO5 media that is causing the issue. Please let us know if you have any other questions.  
Hi, I bought at TL4000 two months ago and everything has worked fine until now. The TL4000 is equipted with 3 drives (IBM ULT3580-HH5 ). Two drives that handles LTO5 tapes and one that handles TLO4 tapes. I use two servers, one with Backup Exec 12.5 installed and one with Backup Exch 2012 with all patches installed. After around 200 hours in use all the LTO5 tapes are marked "End Marker Unreadable". TL4000 has marked all the tapes as Bad Tapes. The drives has been cleaned several times, manually and automatic . Already used 2 cleaning tapes. I haven't seen any problems from the drive handled by BE 12.5.  All drivers, firmware etc are updated with newest build. I have now replaced the LTO5 tapes with a few LTO4 tapes and now I don't have issues with the drives. So I don't believe it's a drive issue. LTO4 tapes are manufactured by Imation Corp and LTO5 tapes are manufatured by HP So to my question, are there any know issues with TL4000 and LTO5 tapes? And have I wasted a lot of money one useless tapes? Regards Steen  
TL4000 "End Marker Unreadable" on all LTO 5 tapes