[gentoo-user] How to copy gzip data from bytestream?

[Grant Edwards]

I've got a "raw" USB flash drive containing a large chunk of gzipped
data. By "raw" I mean no partition table, now filesystem. Think of it
as a tape (if you're old enough).

gzip -tv is quite happy to validate the data and says it's OK, though
it says it ignored extra bytes after the end of the "file".

The flash drive size is 128GB, but the gzipped data is only maybe
20-30GB.

Question: is there a simple way to copy just the 'gzip' data from the
drive without copying the extra bytes after the end of the 'gzip'
data?

The only thing I can think of is:

   $ zcat /dev/sdX | gzip -c > data.gz

But I was trying to figure out a way to do it without uncompressing
and recompressing the data. I had hoped that the gzip header would
contain a "length" field (so I would know how many bytes to copy using
dd), but it does not. Apparently, the only way to find the end of the
compressed data is to parse it using the proper algorithm (deflate, in
this case).

--
Grant