From Garmin FAQ:
What is the largest size microSD card I can use with my GPSMAP 62 or 78 series device?
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The GPSMAP 62* and 78 series devices have an expandable memory slot into which a microSD** card can be inserted. This card provides the ability to install detailed mapping to enhance the device's internal basemap.

A standard speed card, no larger than 32 GB, can be used in order to get the best performance from the device. Class 4 to Class 10 is considered standard speed.

While considering memory limitations, another limitation is the number of individual mapping segments. Roughly 3,000 mapping segments can be transferred to the GPSMAP 62 or 78 series.

This series will pick and choose which map segments to show when the map segment limit is surpassed. It is suggested to transfer additional mapping segments to another microSD card.


*The GPSMAP 62 does not have a microSD card slot.

I think actually the map segment limit is 4000, and maps supported number with the latest firmware v4.30 should be 22

Oregon FAQ:
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The nüvi units are compatible with both normal Secure Digital (SD) data cards and Secure Digital High Capacity (SDHC) data cards.
16gb SD card is confirmed to work in Garmin 265WT,Nuvi 350,Nuvi ,Zumo 660, Nuvi 1460,Nuvi 760, 62s
32 GB, Class 10, in the case of 3490-