Out of the install manual.
5. The GMA 245 will begin checking the SD card for updated software.
a) Do not turn the GMA 245 off or remove power during the update process.
b) The top row of keys, from left to right, will indicate the progress of the update process.
c) If a problem is encountered during the update process, all keys on the top row will flash at a 1 Hz rate.
6. If a problem is encountered during the update process, follow the below steps for possible solutions.
a) Use the Pilot Knob to power cycle the GMA 245, then perform the process again.
b) Attempt to re-download the GMA 245 software to the PC, recreating the loader SD card, and perform the process again.
c) Recreate the loader card using the recommended Sandisk® brand 4GB SD card, and attempt the update process again.
d) If still having difficulty performing the update, contact Garmin Aviation Technical support.
OK, in step 5(b), it says blinking left to right. You are correct in that. You are getting step 5(c), which means a problem updating.
You have tried doing step 6(a) and (b), so first are you using the recommended ScanDisk 4GB SD card? If not, at least try another 4GB SD card, then a ScanDisk branded SD card. Sometimes a single SD card can be PITA.
If not, then try step 6(d) and contact Garmin Tech support.