I think it is possible you are getting UBL's Mesh Commands confused with RC-8's Bi-Linear Mesh commands. In the UBL system you can save the mesh in slot # 1 by doing a G29 S1. But in RC-8's Bi_linear bed leveling things are documented as:
If you are using the UBL system... Very soon there will be a version of it built on RC-8. Right now, UBL is built on RC-7 and forked on July 4th from that code base.
Quote
/**
* G29: Mesh-based Z probe, probes a grid and produces a
* mesh to compensate for variable bed height
*
* Parameters With MESH_BED_LEVELING:
*
* S0 Produce a mesh report
* S1 Start probing mesh points
* S2 Probe the next mesh point
* S3 Xn Yn Zn.nn Manually modify a single point
* S4 Zn.nn Set z offset. Positive away from bed, negative closer to bed.
* S5 Reset and disable mesh
*
If you are using the UBL system... Very soon there will be a version of it built on RC-8. Right now, UBL is built on RC-7 and forked on July 4th from that code base.
