Or does it work this way:

Amanda sends compressed data to the tape drive.

The tape drive also compresses the data, and therefore actually expands it. 
Amanda doesn't know this.

After Amanda has sent 135GB to the tape drive, the tape drive has actually
written 160GB to the tape, and tells Amanda that it's full.

Amand therefore thinks the tape drive's capacity is only 135GB.

Is that correct?

So if I use this tapetype definition, will Amanda in future only ever send
135GB to the drive?

If I disable hardware compression, then I will always get a minimum of 135GB
worth of capacity.  If I modify the tapetype definition to contain, for
example 155000 mbytes, then I will get a minimum of approximately 150GB
worth of capacity, but depending on how compressable the data is, it may in
fact be a lot more than that.

Does that make sense?

Furthermore, if Amanda can be configured to send uncompressed data to the
drive, and the drive has hardware compression on, then I could expect a
capacity of around 320GB, depending on how compressable the data is?

My head hurts...
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