If the external drive is formatted, most probably, with Apple's utility, it has an interleave factor of 3:1 for the Plus. The SE uses an interleave of 2:1. The ratios reflect the number of revolutions of a platter needed to read a complete track in the first Macs to have SCSI, and is necessary because the port speeds are 312kB/s and ~650kB/s. The SE is expecting data at twice the speed that the Plus can deliver.
If you have the original HDD in the external enclosure, possibly a Rodime 3.5-in or a Seagate 5.25-in, even reformatting the drives from a Mac that is capable of 1:1 interleave with an updated disk driver, such as the Mac II, may not get around the problem. If the present data can be saved elsewhere, you might care to try the reformatting path. A more modern drive (one of the smaller Quantum Pro ELS or LPS units) instead of whatever is in the enclosure now might just save the day for you.
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If you have the original HDD in the external enclosure, possibly a Rodime 3.5-in or a Seagate 5.25-in, even reformatting the drives from a Mac that is capable of 1:1 interleave with an updated disk driver, such as the Mac II, may not get around the problem. If the present data can be saved elsewhere, you might care to try the reformatting path. A more modern drive (one of the smaller Quantum Pro ELS or LPS units) instead of whatever is in the enclosure now might just save the day for you.
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