Anyone that's never had the (dis)pleasure of working with tapes, there's some gotchas to know about before archiving and forgetting about it.
When you create an archive, do not skip the step of reading it back. I'm not talking about just having the system confirm it is readable. I mean actually restoring the data from the tape. I've had tapes fail to retrieve data off of the tape even though the writing app confirmed it was readable.
Drives can have issues reading data written by another drive. Drives can have issues reading a tape it wrote itself.
Why write one when you can write two at twice the time/cost? In all of my experience with tape, I've never written just one tape. In the early days of DVDs, the images were written to LTO to send to have the discs pressed. Two were always sent on the chance one wasn't readable and schedules were so tight that the loss of a couple of days to send a new tape would wreck delivery dates. Another facility wrote three tapes: one kept on site, one sent to IronMountain, one sent to the client directly. Plenty of occasions saw the local copy unreadable and the IronMountain copy was retrieved. These were managed by large robotic tape libraries so each tape was written in a different drive at the same time so it was 3x cost while only 1x time.
Use the drive cleaning cassette as recommended. Your dirty heads can cause the data to not be written/read correctly depending on which head is dirty. If you're having issues reading, clean the heads before giving up on the tape. If you're having issues reading a tape just written, do not think someone else will be able to read it.
Ensure whoever might be using a tape you've written understands what was used to write the tape. Not all archivers are compatible. If you use the tar command to write to tape, people using a GUI app won't be able to read it.
Is there some reason that the data is not written to a section of tape using one head, and then read off of that same tape 1 inch (hundreds of milliseconds? tens?) later using a separate head for verification, before it's tucked back into the spool?
From your experience are any of the big cloud providers more effective/careful at this than others?
Are there any recommended alternatives to IronMountain nowadays?
Symantec Backup Exec was genuinely one of the worst pieces of software I have ever had to use. Later in life I got to use the IBM Storage Archive setup with much larger tape libraries which still sucked but my god the level of competency difference between the two applications was insane.
I have worked with 5 different LTO tapes versions, 5 different other cassette formats (DAT, DDS, etc), and open spools… never, ever, ever in the 15 years had ANY problem (I’ve probably used 1000 of them). We did cycle them very often, we did have proper maintenance of the drives, through. Now that, compared with RAID Disk arrays, where we had monthly failures, was much better. Also I would not trust the „semi-permanent“ NVMEs, where after 1 year without power they get basically reset