The original GB map file on the Adventurer (SD card) would have been in .QC3 format (with a small .QCT "sidecar" file) but on your PC the map is probably in a single .QCT format file (~3GB). The Adventurer can read .QC3 files up to the filing system limit (4GB) but .QCT files only up to about 400MB. If you have somehow "lost" the original .QC3 file on the Adventurer, you will either have to try to get it replaced by technical support or "cut" the PC's .QCT map file into sections of less than 400MB as described on page 7 of the user manual posted above.
This is the best advice. The best way to tell if the map is an older (non-encrypted) map is the QCT file. If the QCT file is roughly 18kb in size, it should have an associated QC3 file of much larger size. The older maps (only QCT) would be contained in one file and would be larger than 18kb. I would delete the 89Mb file and try to copy again. I would certainly contact UK tech support (firstname.lastname@example.org
) about possibly reloading the SD card.