I'm afraid the eTrex does not have the ability to see multiple GMAPSUPP.IMG files.You need to compile all the map tiles into single mapset in a single .IMG file.
Legend HCx can support 8GB sd and maps greater than 2 GB since software version 3.00. As long as sd is formatted in FAT32 it restricts individual file sizes to less than 4GB.
SD/Garmin/gmapsupp.img
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