Why is my duplicate job smaller than the original backup job?

Strange one. Our full backup and duplicate of that backup are different sizes, I'm trying to identify why...

Full backup details:
Backup to HP tape drive
Total size 435GB
Compression: Software
Encrypted: Yes

Duplicate details:
Duplicated from HP tape (above) to external USB hard drive
Compression: Hardware [if available, otherwise none]
Total size: 307GB

Any suggestions? Thanks...
Chris.
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ChrisCranieAsked:
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honmapogConnect With a Mentor Commented:
You're having this because you use software compression.
The 435GB of the original job is the size of the data uncompressed.
BE compresses it down to 307GB of data into the Backup to Disk folder. That compressed 307Gb is then duplcated to tape.

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Gerald ConnollyConnect With a Mentor Commented:
Chris,
What are the actual sizes of the original, on tape and on USB Harddrive?

One of the obvious reasons is the 1000*1000 v 1024*1024  issue, that always crops up when discussing storage sizes (its an approx 7% difference), but it could also be down to Blocksize and also allocation size.

Even looking at the same device via different tools will give different sizes, which is normally down to the 7% diff problem above and/or rounding

G
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ChrisCranieAuthor Commented:
Thanks all.
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