Error Installing Windows 10 via Bootcamp.

When you see the error message, do you also see an OSXRESERVED partition on your disk? If yes, run

diskutil info SomePartition

where SomePartition points to OSXRESERVED. It should be of the form disk0sN, where N is a number.

Can you run SMC and NVRAM reset after disconnecting all external peripherals, and retry BC Assistant? You may have APFS corruption, if you still see the error.

Different poster, same issue. This is the output of `diskutil info xxx`

Device Identifier: disk0s3

Device Node: /dev/disk0s3

Whole: No

Part of Whole: disk0

Volume Name: OSXRESERVED

Mounted: No

Partition Type: Microsoft Basic Data

File System Personality: ExFAT

Type (Bundle): exfat

Name (User Visible): ExFAT

OS Can Be Installed: No

Media Type: Generic

Protocol: PCI-Express

SMART Status: Verified

Volume UUID: XXX

Disk / Partition UUID: XXX

Partition Offset: 384314638336 Bytes (93826816 4096-Byte-Device-Blocks)

Disk Size: 10.0 GB (9999220736 Bytes) (exactly 19529728 512-Byte-Units)

Device Block Size: 4096 Bytes

Volume Total Space: 0 B (0 Bytes) (exactly 0 512-Byte-Units)

Volume Free Space: 0 B (0 Bytes) (exactly 0 512-Byte-Units)

Read-Only Media: No

Read-Only Volume: Not applicable (not mounted)

Device Location: Internal

Removable Media: Fixed

Solid State: Yes

Hardware AES Support: Yes

Error message: "An error occurred while copying the Windows installation files"

System specs:

Model Name: MacBook Pro

Model Identifier: MacBookPro15,1

Processor Name: 8-Core Intel Core i9

Processor Speed: 2.3 GHz

Number of Processors: 1

Total Number of Cores: 8

L2 Cache (per Core): 256 KB

L3 Cache: 16 MB

Hyper-Threading Technology: Enabled

Memory: 16 GB

Boot ROM Version: 1554.80.3.0.0 (iBridge: 18.16.14347.0.0,0)

Serial Number (system): X

Hardware UUID: X

Activation Lock Status: Enabled

This is correctly configured as exFAT to accommodate W10 1903 and later ISOs. Can you post the exact error message?