course backup failing with gigantic checksum 4.9m characters !

You mention 4Gig ... I wonder ... go to Server -> PHP info.

What do you see for Architecture?

Server Environment

PHP variants: Only 64-bit versions of PHP are supported. Note: Changed since 4.1.

Also for MySQL System Variables
temptable_max_ram  1073741824
is 8.5 Gig.

'SoS', Ken