One of the custom chip on the Amiga had hardware for teh track decoding. So the Amiga could read the disk while performing other operations, the data would come in via DMA (direct memopry access) of the co-processor.
(MacOS used variable-speed, that is how it fitted more sectors on outer tracks and achieved 800K.)
If memory serves correct, the raw data was pulled in from disk to memory DMA-style, but the actual decoding could be done either using the blitter or 68k.
True about performing operations whilst loading from disk though; I coded a trackloader demo that was loading the next part of the demo whilst the current part was ongoing, using the 68k to decode so that the blitter could be hammered for graphic effects!
sadly no, it was just deserializing head magnetic flux data (encoded as pulses) to raw bytestream, you still had to compute actual data out of it. Whats worse Commodore shipped same, outdated even in 1987, Double Density chip for the whole life of Amiga.
The fun trick is, with a drive controller which works based on "time between pulses" (DiscFerret, Catweasel, etc) you can do all of the above with a CAV disk drive just by tweaking the read/write clock. Same trick probably works for discrete drive controllers like the 765 and 1771 (I say probably because I haven't checked the datasheet!).
http://amigadev.elowar.com/read/ADCD_2.1/Devices_Manual_guid...
It read full tracks at a time, but you can see from the doc there were 11/22 sectors with no inter sector gaps, but there are separate sectors of 512 bytes which are addressed in the file system structures.
If you decode $4489 via MFM then re-encode that byte, you'll get a 1-bit difference. This is why it works as a sync marker: even if you wrote that byte in the data area of the sector, it wouldn't encode the same way because of the missing clock :)
It's a common trick on radio systems too -- a short burst of data which can't be obtained through normal encoding processes (invalid FEC bits, flipped parity, etc).
One of the HD manufacturers of the time, either Conner or Maxtor, was known to use Amiga 3000 machines to test their SCSI disks, because of the CPU/chipset/OS/FS combination. Not many systems were able to extract 90%+ of theoretical throughput like the A3000 could.
Can it be that the Amiga 500 had in its case a place for a HDD? (I don't remember using external cases, but maybe I have forgotten...)
There was no room in the A500 for a 3.5" drive. Later, the A1200 and A600 had a built-in IDE controller and an internal bay, but for a 2.5" disk.
http://amiga.resource.cx/photos/photos/a590-5.jpg
I only ever saw them with SCSI drives in person, but they also sold XT units. Maybe you saw the latter?