I have an Ultrium 448 tape drive, LTO-2 tapes (200/400GB) and I'm using HP Data Protector as the backup client. The system is set to format and overwrite all tapes but I never manage to get more that ~150GB on a tape before it asks for another tape to be inserted.
Any ideas why this may be happening?
200 GB (marked on the tape) = 200,000,000,000 bytes = ±150 GB (real, 2^30) usable + some metadata.
If you use backup app that has own compression, then there's no hardware compression working with your tape drive, so 200 -> 400 doesn't happen.
IMHO everything works as expected.
As BaronSamedi1958 said, if you are counting binary gigabytes (GiB) then the capacity of a 200 GB tape is about 186 GiB (200 / 1.0243). This is why I encourage people to use real gigabytes (1,000,000,000 bytes) everywhere except when buying RAM.
However, short tapes occur for a number of reasons.
If the tape or drive is dirty, then some blocks will fail to write. The drive will simply continue and write the block in the next available space on the tape. This will result in degraded capacity.
If you cannot supply data to the drive at the full write speed, this can also result in degraded capacity. However, LTO-2 is only 40 MB/s so one can hope that this isn't your problem.
Make sure the drive has been cleaned recently and use a fresh tape. See if the problem persists. This could also indicate a fault in the drive.
On the other hand, tape is cheap and the best solution might be to simply ignore the problem and live with 150 GB per tape.
Also take in account the blocking factor. If you're using for instance a 128K block size, and you're backing up many small files, as each file occupy at least one block on tape, you end up with a lot of wasted space. Typical disk block size is 4K; on tape for decent performance you'll rarely use less than 32K.
I do not have any experience with LTO-2, but I have seen a lot of LTO-4 drives fail in a fashion that would cause a sudden 25% drop in tape capacity. Since you appear to also see exactly 25% drop in tape capacity it is possible that your LTO-2 drive is failing in the same way as the LTO-4 drives I have seen.
An LTO-4 drive will write tracks on the tape in alternating directions. A total of 28 tracks in each direction will be written before the tape is full, and each pass from end to end takes approximately 2 minutes.
The drive will read back all the data it has just written and in case the quality is not good enough the firmware will write another copy transparently to the backup software. This will lose some capacity but protect against many cases of data loss.
However as the drive heads wear down it will reach a point where the firmware will always need to write two copies of every block written, but only in one direction.
Careful measurements of the writing speed would show alternation between the nominal 120MB/s in one direction and 60MB/s in the other direction. If you can feed data to the drive fast enough to ensure that it is always the drive which is the bottleneck, then this distinctive pattern in writing speed is the most reliable way to detect a worn down tape head.